Sound pick-up device



Dec. 27, 1932. H OLSON ET AL 1,892,645

SOUND PICK-UP DEVICE Fi led May 20, 1952 s Sheets-Sheet 1 iNVENTORSHARRY FT OLSON ATTORNEY Dec. 27, 1932.

H. F. OLSON ET AL SOUND PICK-UP DEVICE Filed May 20, 1932 FREQUENCYFREQUENCY Th''OREf/CAL 3 Sheets-Sheet 2 60 3 a 100 t 152% 3. g r 40 2 34 nus 2 FREOULWCY mmflmm; o o o EXfffl/MFNML INVENTORS HARRY F. OLSON 1BY 7%mw ATTORN EY Dec. 27, 1932. OLSON ET L 1,892,645

SOUND PICK-UP DEVICE Filed May 20, 1932 3 Sheets-Sheet 3 ammo/mcm/mrm/sr/c COME/M4770 56 cos. 0)

0 760 lower/om; mmcrmsr/c o/nscr/o/m c/MMUm/sr/c mam: GRID/6W7MICROPHONE PRESSURE MICROPHONE M i=5 cos a 5 =5: 7

INVENTORS HARRY F. OLSON no A '0 NEY Patented Dec. 27, 1932 UNITEDSTATES- PATENT OFFICE HARRY r. OLSON, or conrrnoswoop, AND mm. wnmnnncm,or ammonium,- NEW JERSEY, ASSIGNORS T RADIO CORPORATION or AMERICA, Aoonrom'rron OF DELAWARE SOUND PICK-UP DEvIcE' RElSSUED ApplicationmedKay 20, 1932. Serial No. 812,466.

Our invention relatesto sound pickup devices such as ribbon microphones,andhas for its principal object the provision of an improved pickupdevice which is capable of collecting sound from a predetermined rangeof directions and excluding sound not originating within this range.

A further object is the provision of a microphone which responds both tothe pressure component and to the velocity or pressure gradientcomponent of the sound wave.

Referring to the drawings,

Fig. 1 is afront view of a microphone constructedin accordance with ourinvention, v 7

Figs; 2, 3, and 4 are,, respectively, side, back and bottom views ofthis microphone,

Fig. 5 illustrates the electrical connections of the microphone,

Figs. 6 to 8 illustrate different characteristics of a velocity orpressure gradient microphone,

Figs. 9m 11 illustrate similarcharacteristics of a microphone responsiveto the pressure component of .a sound wave,

Fig. 12 illustrates the directional characteristic of. the pressuregradient microphone, the pressure microphone and the microphoneillustrated by Figures 1 to.5, and

Fig. 13 illustrates the observed directional characteristics of theimproved microphone.

In order that sound radiation may be pro- ,jected from one point toanother point or area with a maximum of efiiciency and a minimum ofinterference from reflecting surfaces directional sound radiators havebeen almost universally employed for sound sources in large scalereproduction of sound. A similar directivity has been found to bedesirable in the sound pickup system to improve the ratio of direct togenerally reflected sound and to otherwise discriminate againstundesirable sounds.

One of the important factors in a directive sound pickup system is thesolid angle over which sound is received without appreciableattenuation. enough to include the average area of action. At thesame-time the angle'should be sufiiciently small so that an appreciablegain in discrimination against undesirable sounds is obtained. Anotherrequirement is a direct onal characteristic which is independent of the,frequency. A system which does not possess this characteristic willproduce frequency discrimination. Due to the large frequen cy band ofthe audible spectrum the use of directional systems which depend uponbetween the normal to the ribbon and the direction of propagation of theincident sound. The ribbon microphone is a pressure grad ent microphoneand its response corresponds to the velocity component of a sound wave.The combination of this type of microphone with a microphone whoseresponse corresponds to the pressure component of a sound wave resultsin the uni-directional microphone hereinafter described in connectionwith Figs. 1 to 5.

The pressure gradient ribbon microphone consists of a light corrugatedmetallic ribbon suspended in a magnetic field and freely accessible tosound vibrations from both sides. The vibration of the ribbon due to animpressed sound wave leads to the induction of an E. M. F. correspondingto the undulations of the incident sound wave.

This must be large phase that actuates the ribbon in the ribbonmicrophone.

In this analysis we will assume a plane wave sound field. Let thepressure at the front of the ribbon be (1) p=Kc A sin ck?) where Kwavelength AX acoustic path between the two sides of the nbbon, C=velocity of propagation A= amplitude of and q5= velocity potential.

The pressure at the back of the ribbon is 2 P=Kc A sin K(a+ sin Kct sinsin(Kct+- cos The generated E. M. F. induced by the motion of the ribbonis given by (5) E Bl X BG where B=flux density Z =length of the ribbon.

=area of ribbon.

The F. generated by the ribbon computed from the equation is shown inFig. 7. As will be seen the experimental results agree with thetheoretically predicted response.

The above considerations have been concerned with the direction ofpropagation normal to the plane of the ribbon. When-the normal to theface of the microphone is inclined by an angle 0 to the, line ofpropagation the acoustic path from the front to the back of the ribbonis multiplied by a factor cos 0. When 0 is 90 the pressure differencebetween the two sides is zero and the ribbon remains stationary. Theobserved direc tional characteristics of this microphone are shown inFig. 8.

The velocity of the ribbon from Equation The resultant pressure on theribbon will be the difference in pressure between the two sides and isgiven by the expression (3) Ag) 2Kc A cos (Kat) sin (2 The velocity ofthe ribbon is given by 1 0 (4) X zw .40

where AipS =the total difference in pressure acting upon the ribbon X=the reactance due to the mass of the ribbon Z =the impedance due to theair load upon the ribbon S =area of the ribbon. ZAGZRAG+7IXAG The ribbonis spaced by a few mils from the pole pieces of the magnetic structure.This aperture gives rise to a mechanical impedance. In general theimpedance due to the spacing is large compared to the mass reactance ofthe ribbon and may be neglected; The reactance X due to the mass of theribbon and the components of the air load Z are shown in Fig. 6.

(4) can be written sin cos 0 na X40 RAG 0=angle between the direction ofpropagation and the normal to the plane of the ribbon.

where The phase angle between the pressure at X =0 and Equation 6 aboveis shown in Fig. 6.

The response of the pressure gradient ribbon microphone described aboveis a measure of the velocity component in a sound wave. By a suitablemodification this instrument may be adapted to respond to the pressurecomponent in a sound wave. One way in which this may be accomplishedwill be described.

In this mechanical system the velocity is given by where XI= velocityP=sound pressure Sp=area of the ribbon Z =the total mechanical impedanceThe generated E. M. F. induced by the motion of the ribbon is given bywhereB=flux density Z =length of the ribbon If the impedance Z is realand independent of the frequency the induced E. M. F. will beindependent of the frequency.

To adapt the ribbon microphone to pressure operation the back side ofthe ribbon is en closed and, terminated in a mechanical resistance whichis large compared to the reactive components. The impedance of theentire mechanical system is given by where X =mass reactance of theribbon, R +iX air load upon the open side of the ribbon, R =resistanceterminating the back of c the ribbon.

As in the case of the pressure gradient microphone the impedance due tospace between the ribbon and the pole pieces may be neglected. Equation9 shows that to maintain constant velocity in this system R must be madelarge compared to R +i(X -l-X This can be accomplished by-proper choiceof the resistance R I The values of Xnr, X, R and R for a particularmicrophone are shown in Fig. 9. As will be seen the resistive componentis large compared. to the reactive component. The phase angle betweenthe velocity of the ribbon and the pressure is shown in Fig. 9.

The E. M. F. generated by the ribbon computed from Equation 8 is shownin Fig. 10. As will be seen the experimental results agree with thetheoretically predicted response.

A true pressure measuring instrument should not discriminate againstanydirection. To attain this objective in any pressure operatedmicrophone thedimensions of the component parts must be made smallcompared to the wave length of the sound wave. This can be accomplishedin the ribbon type of microphone by making the field structure open orwell ventilated.- The directional characteristics of this microphone areshown in Fig. 11. These results indicate that the response isindependent of the direction up to 3000 cycles. Above this frequency thepressure at face of the ribbon is greater than that in free space for0=0. This results in a slight increase in the response above thisfrequency and as a consequence there is a deviation from uniformresponse in all directions above 3000 cycles.

In the preceding discussion we have considered two types of ribbonmicrophones, namely, a microphone'in which the response is a measure ofthe velocity component in the sound wave, and a' microphone in which theresponse is a measure of the pressure in the sound wave.

By a suitable combination of the pressure microphone and the velocity orpressure gradient microphone a uni-directional microphoneis produced.Such a combination is illustrated by Figures 1 to 5. This combinationincludes a corrugated ribbon 10 interposed between pole pieces 11 and 12of a mag net 13 provided with a field coil 14. The ribbon 10 issupported in the magnetic field produced between the pole pieces 11 and12, a member 15 being attached to its upper end and a' member 16 beingattached to its lower end. The current generated by the microphone istransmitted through terminals (not shown) connected to its oppositeends. It will be noted that the pole pieces 11 and 12 are provided withventilating slots 17 and 18 and that a pipe or conduit 19 is provided atits lower end with an enlarged opening which is mounted on the back ofthe device and covers the upper part of the ribbon 10.

With this construction the lower part of the ribbon responds to thevelocity or pressure gradient component of the sound wave and the upperpart of the ribbon responds to the pressure component of the sound wave.Theoretically, the pipe 19 should be of indefinite length to be anacoustic resistance. This, of course is impossible. Substantially thesame result is produced by a pipe filled with loosely packed felt forpreventing reflections from its open end. Thus, with a pipe about threefeet long the requirements of an acoustic resistance of appropriate sizeis obtained. As indicated by Fig. 5, the single ribbon 10 of thepressure and velocity or pressure gradient microphone components may beconnected in series to the input transformer 20 of an amplifier 21.

With the ribbons in the two microphones connected in series the combinedgenerated E. M. F. is given by B S B A 0 a? cos 0 the axis of revolutionnormal to the plane of the ribbons. This is illustrated graphically inFig. 12.

The observed directional characteristics of the combination .are shownin Fig. 8. It will be seen that these directional characteristics arepractically cardioids of revolution up to r rived. The voltage output ofthe uni-directional microphone .for sound originating in the direction 6is The output of a non-directional microphone for sound originating inany direction is END=2E0 This shows that the two microphones have thesame sensitivity for =0.

The efiiciency of energy response of the uni-directional microphone forsounds originating in random directions, all directions being equallyprobable, is

m =O The following conclusion can be drawn: The energy response of theuni-directional microphone to sound originating in random directions isone-third that of a non-directional microphone. For the same allowablereverberation the uni-directional microphone T 27rE f (1 cos 6) sin d0COlvcan be used at 1.7 the distance of a non-directional microphone.

The large solid angle over which this microphone receives sound withoutappreciable attenuation indicates that practically any action can becovered with a single microphone.

Referring to Fig. 12 it will be seen that for angles larger than 90 theresponse is relatively small. In general, undesirable sounds such ascamera noise will fal win this region. Therefore, the particulardirectional characteristicsexhibited bythis microphone will be foundvery useful in overcoming undesirable sounds in sound motion picturerecording or broadcast sound pickup, where desired sounds originate infront'and undesired sounds generally to the rear of the microphone.

Having thus described our invention, what we claim is: I

1. A microphone including a single means one portion of which isresponsive to the pressure gradient of a sound wave and another portionoi which is responsive to the pressure of said wave.

2. A microphone including a ribbon one part of which is responsive tothe velocity of a sound wave and another part of which is responsive tothe pressure of said wave.

3. The combination of means for producing a magnetic field, an elongatedconductor mounted in said field, and means arranged to render only apart of said conductor responsive to the pressure gradient of a soundwave.

4. The combination of means for producing a magnetic field, an elongatedconductor mounted in said field, and means including a pipe in closeproximity to a part oi said conductor for rendering said part responsiveto the pressure gradient of a sound wave.

5. The combination of means for producing a magnetic field, an elongatedconductor mounted in said field, and means including a pipe containingan anti-reflecting material and in close proximity to'a part of saidconductor for rendering said part responsive to the pressure gradient ofa sound wave.

6. The combination of means for producing a magnetic field, an elongatedconductor mounted in said field, and means including a pipe looselypacked with felt and in close proximity to a part of said conductor forrendering said part responsive to the pressure gradient of a'sound wave.

7. The combination of means for producing a magnetic field, an elongatedconductor mounted in said field, and an acoustic means adjacent a partof said conductor for rendering said part responsive to the pressuregradient' of a sound wave.

8. The combination of means for producing amagnetic field, an elongatedconductor mounted in said field, means arranged to render only a part ofsaid conductor responsive to the pressure gradient of a sound wave, andelectrical amplifying means connected to the opposite ends of saidconductor.

HARRY F. OLSON. JULIUS WEINBERGER.

